Falling is a common problem among elderly adults. Each year about half the residents of nursing homes experience a fall, and of those individuals about 15% are seriously injured. While there are many factors that may be associated with falling, deterioration of the postural control system is a serious risk factor for falling. This proposal is designed to provide evidence with regard to the following five questions: 1) whether linear models are sufficient to capture the behavior of the postural control system, 2) whether the assumption of static equilibrium is supported by the behavior of the postural control system, 3) whether center of pressure (COP) and head movement can be used interchangeably, 4) whether there exist resonant frequencies in the postural control system, and 5) whether prospective adjustment in visually guided postural control is related to resonant frequencies in the system. Each of these questions is likely to exhibit age-related differences that may be diagnostic of reduced postural control in aging and the attendant increased risk of falling in older adults. There are seven specific aims that are addressed by this research project: 1) constrain the choice of models for postural control, 2) test whether kinematic and COP measurements can be used interchangeably, 3) provide evidence against either the static or dynamic equilibrium hypotheses, 4) test for age-related differences in the first 3 aims, 5) generate a young-adult baseline set of observations and develop and test models of postural control in these data, 6) test for both parametric and structural age-related differences in these models, and 7) prepare and archive the data for data-sharing and longitudinal follow-up.